1. Field of the Invention
The present invention relates to a laser treatment apparatus for crystallization or activation after ion implantation, of a semiconductor substrate, a semiconductor film or the like using a laser beam and to a method of manufacturing a semiconductor device using such a laser treatment apparatus.
2. Description of the Related Art
Laser annealing is performed to crystallize a semiconductor wafer or a non-single crystalline semiconductor film in a semiconductor manufacturing process or to recover the crystallinity after ion implantation. A conventional laser annealing method involves a method of uniformly irradiating the entire surface of an object with a laser beam as is disclosed in JP 2-181419 A, a method of scanning a spot-shaped beam as is disclosed in JP 62-104117 A, or a method of irradiating a linearly processed beam obtained by an optical system as in a laser treatment apparatus disclosed in JP 8-195357 A.
The term xe2x80x9claser annealingxe2x80x9d herein indicates a technique for recrystallizing a damaged layer or an amorphous layer formed in a semiconductor substrate or a semiconductor film or a technique for crystallizing an amorphous semiconductor film formed on a substrate. The xe2x80x9claser annealingxe2x80x9d also includes a technique that is applied to leveling or improvement of a surface quality of the semiconductor substrate or the semiconductor film. Applicable laser oscillation devices are: gas laser oscillation devices represented by an excimer laser; and solid laser oscillation devices represented by a YAG laser. Such laser oscillation devices are known to heat a surface layer of a semiconductor by laser beam irradiation for an extremely short period of time, i.e., about several tens to several hundreds of nanoseconds so as to crystallize the surface layer.
For example, the above-cited JP 62-104117 A discloses a technique of polycrystallizing an amorphous semiconductor film with a laser beam scanning speed set so as to be beam spot diameterxc3x975000/seconds or higher, without bringing the amorphous semiconductor film into a completely melted state. U.S. Pat. No. 4,330,363 B discloses a technique of irradiating a semiconductor region formed in an island-like shape with an elongated laser beam to substantially form a single-crystalline region.
The laser annealing is characteristic in that a treatment time period can be remarkably reduced as compared with annealing utilizing radiant heating or thermal conductive heating, that a semiconductor or a semiconductor film is selectively and locally heated so that a substrate is scarcely thermally damaged, and the like.
Recently, the laser annealing has been positively utilized for forming a polycrystalline silicon film on a glass substrate. This formation process is applied to the manufacture of a thin film transistor (TFT) which is used as a switching element of a liquid crystal display device. Since only a region where a semiconductor film is formed is thermally affected with the use of an excimer laser, the laser annealing with the excimer laser allows the use of a cheap glass substrate, thereby realizing the application of a glass substrate to a large display.
Moreover, since a TFT, which is manufactured of a polycrystalline silicon film crystallized by the laser annealing, can be driven at a relatively high frequency, not only a switching element provided at a pixel but also a driver circuit can be formed on a glass substrate. A pattern design rule is about 5 to 20 xcexcm. Thus, about 106 to 107 TFTs are formed in a driver circuit and a pixel portion on the glass substrate, respectively.
The crystallization of amorphous silicon by laser annealing is achieved through a process of melt-solidification. More specifically, this process is regarded as a two-stage process consisting of: the generation of crystal nuclei; and the crystal growth from the crystal nuclei. However, the laser annealing using a pulsed laser beam is not capable of controlling the positions where crystal nuclei are formed and the density of the formed crystal nuclei, leaving the crystal nuclei spontaneously generated. Therefore, crystal grains are formed at arbitrary positions in a plane of a glass substrate. Moreover, the size of the obtained crystal grain is small, i.e., about 0.2 to 0.5 xcexcm. A number of defects are generated at crystal grain boundaries, which is regarded as a primary factor of limiting a field effect mobility of the TFT.
In a technique related to the above-mentioned JP 62-104117 A, however, the crystal growth due to crystal nuclei which are believed to be formed in an unmelted region is dominant because a semiconductor film is not completely melted. As a result, the increase in size of crystal grains cannot be realized. More specifically, substantially single-crystalline crystals cannot be formed over the entire surface of a semiconductor film on which a channel region of a TFT is to be formed.
In the first place, a crystallization method with continuous wave laser scanning while effecting melt-solidification is considered to be close to a zone melting method. Although a high energy density is required to melt a semiconductor, this crystallization method has drawbacks in that it is difficult to realize a high output with the continuous wave laser, resulting in increase in apparatus size. Ultimately, the beam size is reduced through an optical system to be radiated to a semiconductor. With such beam size, however, a considerably long time period for the treatment is required to achieve the crystallization over the entire surface of a large substrate.
A laser beam capable of heating a semiconductor film is present in a wide range over an ultraviolet range to an infrared range. It is preferred to use a laser beam having a wavelength in an ultraviolet to visible range for selectively heating a semiconductor film or a semiconductor region formed on a substrate in view of the relationship with an absorption coefficient of a semiconductor. However, since a laser beam emitted from a solid laser oscillation device exhibits a strong coherence, the interference is caused on the irradiated surface. As a result, a uniform laser beam cannot be radiated.
The present invention is devised in view of the above problems, and has an object to provide a laser treatment apparatus for irradiating the position where a TFT is to be formed with a laser beam over the entire surface of a large substrate to achieve the crystallization, thereby being capable of forming a crystalline semiconductor film having a large grain diameter with high throughput.
In order to achieve the above-mentioned object, according to the present invention, there is provided a laser irradiation apparatus including: a first movable mirror for deflecting a laser beam in a main scanning direction; and an elongated second movable mirror for receiving the laser beam deflected in the main scanning direction and scanning the laser beam in a sub scanning direction, in which the second movable mirror includes means for scanning the laser beam in the sub scanning direction at a rotation angle around an axis in a longitudinal direction of the second movable mirror and irradiating an object to be treated which is placed on a stage with the laser beam.
Also, according to another structure of the present invention, there is provided a laser irradiation apparatus including: a first laser beam scanning system including a first movable mirror for deflecting a laser beam in a first main scanning direction, and an elongated second movable mirror for receiving the laser beam deflected in the first main scanning direction and scanning the laser beam in a first sub scanning direction; and a second laser beam scanning system including a third movable mirror for deflecting a laser beam in a second main scanning direction, and a fourth movable mirror for receiving the laser beam deflected in the second main scanning direction and scanning the laser beam in a second sub scanning direction, in which: the second movable mirror includes means for scanning the laser beam in the first sub scanning direction at a rotation angle round an axis in a longitudinal direction of the second movable mirror and irradiating an object to be treated which is placed on a stage with the laser beam; and the fourth movable mirror includes means for scanning the laser beam in the second sub scanning direction at a rotation angle around an axis in a longitudinal direction of the fourth movable mirror and irradiating the object to be treated which is placed on the stage with the laser beam.
In a preferable embodiment, each of the first and the third movable mirrors can be one of a galvano mirror and a polygon mirror.
As a laser for supplying a laser beam, a solid laser or a gas laser is used.
In the above structure of the invention, a laser beam is scanned in a main scanning direction with a first movable mirror and is scanned in a sub scanning direction with a second movable mirror, thereby allowing a laser beam to be radiated to an arbitrary position on an object to be treated. Moreover, a plurality of laser beam scanning means are provided so as to radiate laser beams from a biaxial direction to a surface to be formed, thereby reducing a laser treatment time period.
A laser treatment apparatus according to the present invention includes: a plurality of optical systems, each including a laser oscillation device and first deflection means for deflecting a laser beam output from the laser oscillation device in a main scanning direction; and second deflection means for receiving a plurality of laser beams which are deflected in the main scanning direction and scanning in a sub scanning direction. Here, the second deflection means has a function of scanning the plurality of laser beams in the sub scanning direction at a rotation angle around an axis in a uniaxial direction of the second deflection means and irradiating an object to be treated which is placed on a stage with the laser beams.
As the first deflection means, a movable mirror whose rotation angle can be arbitrary set is suitable. Typically, a galvano mirror can be used. As the second deflection means, an elongated movable mirror is suitable which has such an area that allows the reception of a plurality of laser beams, and which is rotatable around the axis in a longitudinal direction of the mirror. Using rotation angles of these two mirrors, laser beams irradiating the object to be treated can be radiated to an arbitrary position from the main scanning direction and the sub scanning direction. Moreover, it is possible to scan a laser beam by continuously varying the angles of the two mirrors.
Also, according to another structure of the present invention, there is provided a laser treatment apparatus including: a plurality of optical systems, each including a laser oscillation device and first deflection means for deflecting a laser beam output from the laser oscillation device in a first main scanning direction; a plurality of optical systems, each including second deflection means for receiving a plurality of laser beams which are deflected in the first main scanning direction and scanning in a first sub scanning direction and third deflection means for deflecting the laser beams output from the laser oscillation device in a second main scanning direction; and fourth deflection means for receiving the plurality of laser beams deflected in the second main scanning direction and scanning in a second sub scanning direction. Here, the second and fourth deflection means have a function of scanning the plurality of laser beams in the sub scanning directions at a rotation angle around a uniaxial direction and irradiating an object to be treated which is placed on a stage with the laser beams. With this structure, it is possible to increase the number of laser beams capable of being radiated to and scanning the object to be treated, and thus, the time period necessary for laser treatment can be reduced.
An fxcex8 lens is provided as means for correcting a scanning speed so as to be constant regardless of an irradiation angle, in the structure of the above-described optical system. As the laser oscillation device, a gas laser oscillation device or a solid laser oscillation device is used; in particular, a laser oscillation device operable in a continuous oscillation mode is used. As a solid laser oscillation device operable in a continuous oscillation mode, a laser oscillation device using crystal such as YAG, YVO4, YLF or YAl5O12 doped with Nd, Tm or Ho is used. Although a fundamental wave of an oscillation wavelength varies depending on a dopant, a laser oscillates at a wavelength of 1 to 2 xcexcm. In order to crystallize an amorphous semiconductor film, it is preferred to use a laser beam having a wavelength falling within a visible range to an ultraviolet range and to use a second harmonic wave to a fourth harmonic wave of a fundamental wave in order that the laser beam is selectively absorbed by a semiconductor film. Typically, for crystallization of amorphous silicon, a second harmonic wave (532 nm) of an Nd: YVO4 laser (fundamental wave: 1064 nm) is used. Besides the Nd: YVO4 laser, a gas laser oscillation device such as an argon laser, a krypton laser or an excimer laser can be used.
Since a laser beam emitted from a solid laser oscillation device has strong coherence to cause interference on the irradiated surface, a plurality of laser beams emitted from different laser oscillation devices are superimposed on the irradiated portion as means for canceling the interference. With such a structure, not only interference is eliminated but also a substantial energy density in the irradiated portion can be increased. As alternative means, a plurality of laser beams emitted from different laser oscillation devices may be superimposed on the same optical axis on the midway of the optical system.
A laser treatment apparatus provided with a means for eliminating the interference can be achieved with a structure such that a laser treatment apparatus includes n (n=natural number) optical systems, the n-th optical system including an n-th laser oscillation device, deflection means for operating a laser beam in an n-th Y-axis direction, deflection means for scanning a laser beam in an n-th X-axis direction and an n-th fxcex8 lens, in which n laser beams, which are converged and deflected by the n optical systems, have a structure to irradiate generally the same position of an object to be treated.
Owing to the above-mentioned structure of the laser treatment apparatus, a laser beam having a sufficiently high energy density to melt a semiconductor can be radiated without causing interference in the irradiated portion. Moreover, by scanning a laser beam with deflection means, the crystallization of an amorphous semiconductor film can be achieved over the entire surface of a large substrate.
It is not necessary to radiate a laser beam while scanning the entire surface of the object; it is sufficient to irradiate a specific region with a laser beam while specifying a place to be irradiated. The structure of the laser treatment apparatus of the present invention realizes the targeted irradiation by combining a plurality of movable mirrors. Furthermore, by superimposing the laser beams on the same irradiated portion, the effects of interference can be eliminated.
On the other hand, a method of manufacturing a semiconductor device according to the present invention for solving the above problems, includes the step of scanning a plurality of laser beams in one direction with first deflection means for deflecting the plurality of laser beams in a main scanning direction and second deflection means for scanning in a sub scanning direction, to thereby crystallize a semiconductor film having an amorphous structure formed on an insulating surface or to recover the crystallinity of a semiconductor film. A scanning direction of the laser beam is set identical with a channel length direction of a TFT to lower the probability of the generation of crystal grain boundaries traversing the channel length direction, thereby improving a carrier mobility.
As another structure, a plurality of laser beams are scanned in one direction with first deflection means for deflecting a plurality of first laser beams in a first main scanning direction, second deflection means for scanning in a first sub scanning direction, third deflection means for deflecting a plurality of second laser beams in a second main scanning direction, and fourth deflection means for scanning in a second sub scanning direction to crystallize an amorphous semiconductor film formed on an insulating surface or to recover the crystallinity of a semiconductor film.
A laser beam is output from a solid laser oscillation device in a continuous oscillation mode. In order to selectively heat a semiconductor film, it is preferred that a laser beam falls within a wavelength band having an absorption coefficient of 103 cmxe2x88x921 or higher. For a semiconductor such as silicon or silicon germanium, a laser beam falling within a wavelength band of 700 nm or lower (visible range to ultraviolet range) is desirable.
A region to be irradiated with a laser beam is not required to be the entire surface of a semiconductor film; a laser beam is interruptedly radiated so as to crystallize a selected region of an amorphous semiconductor film or recover the crystallinity of the region.
With the above structure, a laser beam is radiated generally to a position where a TFT is to be formed over the entire surface of a large substrate so as to achieve the crystallization. As a result, a crystalline semiconductor film having a large grain diameter can be obtained with high throughput.
The term xe2x80x9camorphous semiconductor filmxe2x80x9d in the present invention includes, in a narrow sense, not only a semiconductor film having a completely amorphous structure, but also a semiconductor film containing fine crystal particles or a so-called microcrystalline semiconductor film, and a semiconductor film locally having a crystalline structure. Typically, an amorphous silicon film is used. Besides the amorphous silicon film, an amorphous silicon germanium film, an amorphous silicon carbide film and the like can also be used.